Blood is the bodily fluid responsible for transport of materials and waste products throughout the body. It carries oxygen from and carbon dioxide to the lungs, nutrients from the digestive system or storage sites to tissues that require them, and waste products from the tissues to the liver for detoxification and to the kidneys for disposal. Blood delivers hormones to their sites of action and circulates numerous critical parts of the immune system throughout the body. Blood regulates its own pH , as well as that of the intercellular fluid in the body, and aids in thermoregulation by redistributing heat. Blood also carries the proteins and other factors it needs to clot, thereby preventing its own loss in the event of injury to the vessels in which it travels. usually with no nucleus or internal membranes, and are stuffed full of the oxygen-binding protein hemoglobin.
A human adult has 4 to 6 liters (1 to 1.5 gallons) of blood, approximately 92 percent of which is water. Nearly half its volume is red blood cells (RBCs, or erythrocytes). Proteins, sugars, salts, white blood cells, and platelets make up the remainder. The noncellular portion is termed plasma, while the cellular parts are collectively referred to as the formed elements. Blood forms in the bone marrow, a spongy tissue contained in the bones.
Red Blood Cells and Hemoglobin
Only a small amount of the oxygen needed for life can dissolve directly in plasma. Oxygen transport instead relies on red blood cells. At any one time, there are more than 25 trillion RBCs in circulation in an adult, more than the combined total of all other cell types in the body. As RBCs develop, they extrude their cell nucleus , so that at maturity they have almost nothing inside their membranes except the oxygen-carrying protein, hemoglobin . The absence of a nucleus contributes to the RBC's short life, as does the constant physical stress it experiences squeezing through capillaries that are narrower than it is. The average RBC circulates for approximately 120 days before being destroyed in the liver, bone marrow, or spleen. The iron from hemoglobin is recycled, while the cyclic nitrogen compound that holds it, called heme, is converted to bilirubin. Bilirubin is transported to the liver for elimination from the body as bile. Liver disease can cause jaundice, a yellowing of the skin due to bilirubin in the blood.